Factor a bunch of functionality related to memcpy and memset transforms out of

GVN and into its own pass.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@49419 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 769 insertions, 0 deletions

diff --git a/lib/Transforms/Scalar/MemCpyOptimizer.cpp b/lib/Transforms/Scalar/MemCpyOptimizer.cpp
new file mode 100644
index 0000000..f990ba8
--- /dev/null
+++ b/lib/Transforms/Scalar/MemCpyOptimizer.cpp
@@ -0,0 +1,769 @@
+//===- MemCpyOptimizer.cpp - Optimize use of memcpy and friends -----------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// This pass performs various transformations related to eliminating memcpy
+// calls, or transforming sets of stores into memset's.
+//
+//===----------------------------------------------------------------------===//
+
+#define DEBUG_TYPE "memcpyopt"
+#include "llvm/Transforms/Scalar.h"
+#include "llvm/BasicBlock.h"
+#include "llvm/Constants.h"
+#include "llvm/DerivedTypes.h"
+#include "llvm/Function.h"
+#include "llvm/IntrinsicInst.h"
+#include "llvm/Instructions.h"
+#include "llvm/ParameterAttributes.h"
+#include "llvm/Value.h"
+#include "llvm/ADT/DepthFirstIterator.h"
+#include "llvm/ADT/SmallVector.h"
+#include "llvm/ADT/Statistic.h"
+#include "llvm/Analysis/Dominators.h"
+#include "llvm/Analysis/AliasAnalysis.h"
+#include "llvm/Analysis/MemoryDependenceAnalysis.h"
+#include "llvm/Support/CFG.h"
+#include "llvm/Support/CommandLine.h"
+#include "llvm/Support/Compiler.h"
+#include "llvm/Support/Debug.h"
+#include "llvm/Support/GetElementPtrTypeIterator.h"
+#include "llvm/Target/TargetData.h"
+#include <list>
+using namespace llvm;
+
+STATISTIC(NumMemCpyInstr, "Number of memcpy instructions deleted");
+STATISTIC(NumMemSetInfer, "Number of memsets inferred");
+
+namespace {
+  cl::opt<bool>
+  FormMemSet("form-memset-from-stores",
+             cl::desc("Transform straight-line stores to memsets"),
+             cl::init(true), cl::Hidden);
+}
+
+/// isBytewiseValue - If the specified value can be set by repeating the same
+/// byte in memory, return the i8 value that it is represented with.  This is
+/// true for all i8 values obviously, but is also true for i32 0, i32 -1,
+/// i16 0xF0F0, double 0.0 etc.  If the value can't be handled with a repeated
+/// byte store (e.g. i16 0x1234), return null.
+static Value *isBytewiseValue(Value *V) {
+  // All byte-wide stores are splatable, even of arbitrary variables.
+  if (V->getType() == Type::Int8Ty) return V;
+  
+  // Constant float and double values can be handled as integer values if the
+  // corresponding integer value is "byteable".  An important case is 0.0. 
+  if (ConstantFP *CFP = dyn_cast<ConstantFP>(V)) {
+    if (CFP->getType() == Type::FloatTy)
+      V = ConstantExpr::getBitCast(CFP, Type::Int32Ty);
+    if (CFP->getType() == Type::DoubleTy)
+      V = ConstantExpr::getBitCast(CFP, Type::Int64Ty);
+    // Don't handle long double formats, which have strange constraints.
+  }
+  
+  // We can handle constant integers that are power of two in size and a 
+  // multiple of 8 bits.
+  if (ConstantInt *CI = dyn_cast<ConstantInt>(V)) {
+    unsigned Width = CI->getBitWidth();
+    if (isPowerOf2_32(Width) && Width > 8) {
+      // We can handle this value if the recursive binary decomposition is the
+      // same at all levels.
+      APInt Val = CI->getValue();
+      APInt Val2;
+      while (Val.getBitWidth() != 8) {
+        unsigned NextWidth = Val.getBitWidth()/2;
+        Val2  = Val.lshr(NextWidth);
+        Val2.trunc(Val.getBitWidth()/2);
+        Val.trunc(Val.getBitWidth()/2);
+
+        // If the top/bottom halves aren't the same, reject it.
+        if (Val != Val2)
+          return 0;
+      }
+      return ConstantInt::get(Val);
+    }
+  }
+  
+  // Conceptually, we could handle things like:
+  //   %a = zext i8 %X to i16
+  //   %b = shl i16 %a, 8
+  //   %c = or i16 %a, %b
+  // but until there is an example that actually needs this, it doesn't seem
+  // worth worrying about.
+  return 0;
+}
+
+static int64_t GetOffsetFromIndex(const GetElementPtrInst *GEP, unsigned Idx,
+                                  bool &VariableIdxFound, TargetData &TD) {
+  // Skip over the first indices.
+  gep_type_iterator GTI = gep_type_begin(GEP);
+  for (unsigned i = 1; i != Idx; ++i, ++GTI)
+    /*skip along*/;
+  
+  // Compute the offset implied by the rest of the indices.
+  int64_t Offset = 0;
+  for (unsigned i = Idx, e = GEP->getNumOperands(); i != e; ++i, ++GTI) {
+    ConstantInt *OpC = dyn_cast<ConstantInt>(GEP->getOperand(i));
+    if (OpC == 0)
+      return VariableIdxFound = true;
+    if (OpC->isZero()) continue;  // No offset.
+
+    // Handle struct indices, which add their field offset to the pointer.
+    if (const StructType *STy = dyn_cast<StructType>(*GTI)) {
+      Offset += TD.getStructLayout(STy)->getElementOffset(OpC->getZExtValue());
+      continue;
+    }
+    
+    // Otherwise, we have a sequential type like an array or vector.  Multiply
+    // the index by the ElementSize.
+    uint64_t Size = TD.getABITypeSize(GTI.getIndexedType());
+    Offset += Size*OpC->getSExtValue();
+  }
+
+  return Offset;
+}
+
+/// IsPointerOffset - Return true if Ptr1 is provably equal to Ptr2 plus a
+/// constant offset, and return that constant offset.  For example, Ptr1 might
+/// be &A[42], and Ptr2 might be &A[40].  In this case offset would be -8.
+static bool IsPointerOffset(Value *Ptr1, Value *Ptr2, int64_t &Offset,
+                            TargetData &TD) {
+  // Right now we handle the case when Ptr1/Ptr2 are both GEPs with an identical
+  // base.  After that base, they may have some number of common (and
+  // potentially variable) indices.  After that they handle some constant
+  // offset, which determines their offset from each other.  At this point, we
+  // handle no other case.
+  GetElementPtrInst *GEP1 = dyn_cast<GetElementPtrInst>(Ptr1);
+  GetElementPtrInst *GEP2 = dyn_cast<GetElementPtrInst>(Ptr2);
+  if (!GEP1 || !GEP2 || GEP1->getOperand(0) != GEP2->getOperand(0))
+    return false;
+  
+  // Skip any common indices and track the GEP types.
+  unsigned Idx = 1;
+  for (; Idx != GEP1->getNumOperands() && Idx != GEP2->getNumOperands(); ++Idx)
+    if (GEP1->getOperand(Idx) != GEP2->getOperand(Idx))
+      break;
+
+  bool VariableIdxFound = false;
+  int64_t Offset1 = GetOffsetFromIndex(GEP1, Idx, VariableIdxFound, TD);
+  int64_t Offset2 = GetOffsetFromIndex(GEP2, Idx, VariableIdxFound, TD);
+  if (VariableIdxFound) return false;
+  
+  Offset = Offset2-Offset1;
+  return true;
+}
+
+
+/// MemsetRange - Represents a range of memset'd bytes with the ByteVal value.
+/// This allows us to analyze stores like:
+///   store 0 -> P+1
+///   store 0 -> P+0
+///   store 0 -> P+3
+///   store 0 -> P+2
+/// which sometimes happens with stores to arrays of structs etc.  When we see
+/// the first store, we make a range [1, 2).  The second store extends the range
+/// to [0, 2).  The third makes a new range [2, 3).  The fourth store joins the
+/// two ranges into [0, 3) which is memset'able.
+namespace {
+struct MemsetRange {
+  // Start/End - A semi range that describes the span that this range covers.
+  // The range is closed at the start and open at the end: [Start, End).  
+  int64_t Start, End;
+
+  /// StartPtr - The getelementptr instruction that points to the start of the
+  /// range.
+  Value *StartPtr;
+  
+  /// Alignment - The known alignment of the first store.
+  unsigned Alignment;
+  
+  /// TheStores - The actual stores that make up this range.
+  SmallVector<StoreInst*, 16> TheStores;
+  
+  bool isProfitableToUseMemset(const TargetData &TD) const;
+
+};
+} // end anon namespace
+
+bool MemsetRange::isProfitableToUseMemset(const TargetData &TD) const {
+  // If we found more than 8 stores to merge or 64 bytes, use memset.
+  if (TheStores.size() >= 8 || End-Start >= 64) return true;
+  
+  // Assume that the code generator is capable of merging pairs of stores
+  // together if it wants to.
+  if (TheStores.size() <= 2) return false;
+  
+  // If we have fewer than 8 stores, it can still be worthwhile to do this.
+  // For example, merging 4 i8 stores into an i32 store is useful almost always.
+  // However, merging 2 32-bit stores isn't useful on a 32-bit architecture (the
+  // memset will be split into 2 32-bit stores anyway) and doing so can
+  // pessimize the llvm optimizer.
+  //
+  // Since we don't have perfect knowledge here, make some assumptions: assume
+  // the maximum GPR width is the same size as the pointer size and assume that
+  // this width can be stored.  If so, check to see whether we will end up
+  // actually reducing the number of stores used.
+  unsigned Bytes = unsigned(End-Start);
+  unsigned NumPointerStores = Bytes/TD.getPointerSize();
+  
+  // Assume the remaining bytes if any are done a byte at a time.
+  unsigned NumByteStores = Bytes - NumPointerStores*TD.getPointerSize();
+  
+  // If we will reduce the # stores (according to this heuristic), do the
+  // transformation.  This encourages merging 4 x i8 -> i32 and 2 x i16 -> i32
+  // etc.
+  return TheStores.size() > NumPointerStores+NumByteStores;
+}    
+
+
+namespace {
+class MemsetRanges {
+  /// Ranges - A sorted list of the memset ranges.  We use std::list here
+  /// because each element is relatively large and expensive to copy.
+  std::list<MemsetRange> Ranges;
+  typedef std::list<MemsetRange>::iterator range_iterator;
+  TargetData &TD;
+public:
+  MemsetRanges(TargetData &td) : TD(td) {}
+  
+  typedef std::list<MemsetRange>::const_iterator const_iterator;
+  const_iterator begin() const { return Ranges.begin(); }
+  const_iterator end() const { return Ranges.end(); }
+  bool empty() const { return Ranges.empty(); }
+  
+  void addStore(int64_t OffsetFromFirst, StoreInst *SI);
+};
+  
+} // end anon namespace
+
+
+/// addStore - Add a new store to the MemsetRanges data structure.  This adds a
+/// new range for the specified store at the specified offset, merging into
+/// existing ranges as appropriate.
+void MemsetRanges::addStore(int64_t Start, StoreInst *SI) {
+  int64_t End = Start+TD.getTypeStoreSize(SI->getOperand(0)->getType());
+  
+  // Do a linear search of the ranges to see if this can be joined and/or to
+  // find the insertion point in the list.  We keep the ranges sorted for
+  // simplicity here.  This is a linear search of a linked list, which is ugly,
+  // however the number of ranges is limited, so this won't get crazy slow.
+  range_iterator I = Ranges.begin(), E = Ranges.end();
+  
+  while (I != E && Start > I->End)
+    ++I;
+  
+  // We now know that I == E, in which case we didn't find anything to merge
+  // with, or that Start <= I->End.  If End < I->Start or I == E, then we need
+  // to insert a new range.  Handle this now.
+  if (I == E || End < I->Start) {
+    MemsetRange &R = *Ranges.insert(I, MemsetRange());
+    R.Start        = Start;
+    R.End          = End;
+    R.StartPtr     = SI->getPointerOperand();
+    R.Alignment    = SI->getAlignment();
+    R.TheStores.push_back(SI);
+    return;
+  }
+
+  // This store overlaps with I, add it.
+  I->TheStores.push_back(SI);
+  
+  // At this point, we may have an interval that completely contains our store.
+  // If so, just add it to the interval and return.
+  if (I->Start <= Start && I->End >= End)
+    return;
+  
+  // Now we know that Start <= I->End and End >= I->Start so the range overlaps
+  // but is not entirely contained within the range.
+  
+  // See if the range extends the start of the range.  In this case, it couldn't
+  // possibly cause it to join the prior range, because otherwise we would have
+  // stopped on *it*.
+  if (Start < I->Start) {
+    I->Start = Start;
+    I->StartPtr = SI->getPointerOperand();
+  }
+    
+  // Now we know that Start <= I->End and Start >= I->Start (so the startpoint
+  // is in or right at the end of I), and that End >= I->Start.  Extend I out to
+  // End.
+  if (End > I->End) {
+    I->End = End;
+    range_iterator NextI = I;;
+    while (++NextI != E && End >= NextI->Start) {
+      // Merge the range in.
+      I->TheStores.append(NextI->TheStores.begin(), NextI->TheStores.end());
+      if (NextI->End > I->End)
+        I->End = NextI->End;
+      Ranges.erase(NextI);
+      NextI = I;
+    }
+  }
+}
+
+//===----------------------------------------------------------------------===//
+//                         MemCpyOpt Pass
+//===----------------------------------------------------------------------===//
+
+namespace {
+
+  class VISIBILITY_HIDDEN MemCpyOpt : public FunctionPass {
+    bool runOnFunction(Function &F);
+  public:
+    static char ID; // Pass identification, replacement for typeid
+    MemCpyOpt() : FunctionPass((intptr_t)&ID) { }
+
+  private:
+    // This transformation requires dominator postdominator info
+    virtual void getAnalysisUsage(AnalysisUsage &AU) const {
+      AU.setPreservesCFG();
+      AU.addRequired<DominatorTree>();
+      AU.addRequired<MemoryDependenceAnalysis>();
+      AU.addRequired<AliasAnalysis>();
+      AU.addRequired<TargetData>();
+      AU.addPreserved<AliasAnalysis>();
+      AU.addPreserved<MemoryDependenceAnalysis>();
+      AU.addPreserved<TargetData>();
+    }
+  
+    // Helper fuctions
+    bool processInstruction(Instruction* I,
+                            SmallVectorImpl<Instruction*> &toErase);
+    bool processStore(StoreInst *SI, SmallVectorImpl<Instruction*> &toErase);
+    bool processMemCpy(MemCpyInst* M, MemCpyInst* MDep,
+                       SmallVectorImpl<Instruction*> &toErase);
+    bool performCallSlotOptzn(MemCpyInst* cpy, CallInst* C,
+                              SmallVectorImpl<Instruction*> &toErase);
+    bool iterateOnFunction(Function &F);
+  };
+  
+  char MemCpyOpt::ID = 0;
+}
+
+// createMemCpyOptPass - The public interface to this file...
+FunctionPass *llvm::createMemCpyOptPass() { return new MemCpyOpt(); }
+
+static RegisterPass<MemCpyOpt> X("memcpyopt",
+                                 "MemCpy Optimization");
+
+
+
+/// processStore - When GVN is scanning forward over instructions, we look for
+/// some other patterns to fold away.  In particular, this looks for stores to
+/// neighboring locations of memory.  If it sees enough consequtive ones
+/// (currently 4) it attempts to merge them together into a memcpy/memset.
+bool MemCpyOpt::processStore(StoreInst *SI, SmallVectorImpl<Instruction*> &toErase) {
+  if (!FormMemSet) return false;
+  if (SI->isVolatile()) return false;
+  
+  // There are two cases that are interesting for this code to handle: memcpy
+  // and memset.  Right now we only handle memset.
+  
+  // Ensure that the value being stored is something that can be memset'able a
+  // byte at a time like "0" or "-1" or any width, as well as things like
+  // 0xA0A0A0A0 and 0.0.
+  Value *ByteVal = isBytewiseValue(SI->getOperand(0));
+  if (!ByteVal)
+    return false;
+
+  TargetData &TD = getAnalysis<TargetData>();
+  AliasAnalysis &AA = getAnalysis<AliasAnalysis>();
+
+  // Okay, so we now have a single store that can be splatable.  Scan to find
+  // all subsequent stores of the same value to offset from the same pointer.
+  // Join these together into ranges, so we can decide whether contiguous blocks
+  // are stored.
+  MemsetRanges Ranges(TD);
+  
+  Value *StartPtr = SI->getPointerOperand();
+  
+  BasicBlock::iterator BI = SI;
+  for (++BI; !isa<TerminatorInst>(BI); ++BI) {
+    if (isa<CallInst>(BI) || isa<InvokeInst>(BI)) { 
+      // If the call is readnone, ignore it, otherwise bail out.  We don't even
+      // allow readonly here because we don't want something like:
+      // A[1] = 2; strlen(A); A[2] = 2; -> memcpy(A, ...); strlen(A).
+      if (AA.getModRefBehavior(CallSite::get(BI)) ==
+            AliasAnalysis::DoesNotAccessMemory)
+        continue;
+      
+      // TODO: If this is a memset, try to join it in.
+      
+      break;
+    } else if (isa<VAArgInst>(BI) || isa<LoadInst>(BI))
+      break;
+
+    // If this is a non-store instruction it is fine, ignore it.
+    StoreInst *NextStore = dyn_cast<StoreInst>(BI);
+    if (NextStore == 0) continue;
+    
+    // If this is a store, see if we can merge it in.
+    if (NextStore->isVolatile()) break;
+    
+    // Check to see if this stored value is of the same byte-splattable value.
+    if (ByteVal != isBytewiseValue(NextStore->getOperand(0)))
+      break;
+
+    // Check to see if this store is to a constant offset from the start ptr.
+    int64_t Offset;
+    if (!IsPointerOffset(StartPtr, NextStore->getPointerOperand(), Offset, TD))
+      break;
+
+    Ranges.addStore(Offset, NextStore);
+  }
+
+  // If we have no ranges, then we just had a single store with nothing that
+  // could be merged in.  This is a very common case of course.
+  if (Ranges.empty())
+    return false;
+  
+  // If we had at least one store that could be merged in, add the starting
+  // store as well.  We try to avoid this unless there is at least something
+  // interesting as a small compile-time optimization.
+  Ranges.addStore(0, SI);
+
+  
+  Function *MemSetF = 0;
+  
+  // Now that we have full information about ranges, loop over the ranges and
+  // emit memset's for anything big enough to be worthwhile.
+  bool MadeChange = false;
+  for (MemsetRanges::const_iterator I = Ranges.begin(), E = Ranges.end();
+       I != E; ++I) {
+    const MemsetRange &Range = *I;
+
+    if (Range.TheStores.size() == 1) continue;
+    
+    // If it is profitable to lower this range to memset, do so now.
+    if (!Range.isProfitableToUseMemset(TD))
+      continue;
+    
+    // Otherwise, we do want to transform this!  Create a new memset.  We put
+    // the memset right before the first instruction that isn't part of this
+    // memset block.  This ensure that the memset is dominated by any addressing
+    // instruction needed by the start of the block.
+    BasicBlock::iterator InsertPt = BI;
+  
+    if (MemSetF == 0)
+      MemSetF = Intrinsic::getDeclaration(SI->getParent()->getParent()
+                                          ->getParent(), Intrinsic::memset_i64);
+    
+    // Get the starting pointer of the block.
+    StartPtr = Range.StartPtr;
+  
+    // Cast the start ptr to be i8* as memset requires.
+    const Type *i8Ptr = PointerType::getUnqual(Type::Int8Ty);
+    if (StartPtr->getType() != i8Ptr)
+      StartPtr = new BitCastInst(StartPtr, i8Ptr, StartPtr->getNameStart(),
+                                 InsertPt);
+  
+    Value *Ops[] = {
+      StartPtr, ByteVal,   // Start, value
+      ConstantInt::get(Type::Int64Ty, Range.End-Range.Start),  // size
+      ConstantInt::get(Type::Int32Ty, Range.Alignment)   // align
+    };
+    Value *C = CallInst::Create(MemSetF, Ops, Ops+4, "", InsertPt);
+    DEBUG(cerr << "Replace stores:\n";
+          for (unsigned i = 0, e = Range.TheStores.size(); i != e; ++i)
+            cerr << *Range.TheStores[i];
+          cerr << "With: " << *C); C=C;
+  
+    // Zap all the stores.
+    toErase.append(Range.TheStores.begin(), Range.TheStores.end());
+    ++NumMemSetInfer;
+    MadeChange = true;
+  }
+  
+  return MadeChange;
+}
+
+
+/// performCallSlotOptzn - takes a memcpy and a call that it depends on,
+/// and checks for the possibility of a call slot optimization by having
+/// the call write its result directly into the destination of the memcpy.
+bool MemCpyOpt::performCallSlotOptzn(MemCpyInst *cpy, CallInst *C,
+                               SmallVectorImpl<Instruction*> &toErase) {
+  // The general transformation to keep in mind is
+  //
+  //   call @func(..., src, ...)
+  //   memcpy(dest, src, ...)
+  //
+  // ->
+  //
+  //   memcpy(dest, src, ...)
+  //   call @func(..., dest, ...)
+  //
+  // Since moving the memcpy is technically awkward, we additionally check that
+  // src only holds uninitialized values at the moment of the call, meaning that
+  // the memcpy can be discarded rather than moved.
+
+  // Deliberately get the source and destination with bitcasts stripped away,
+  // because we'll need to do type comparisons based on the underlying type.
+  Value* cpyDest = cpy->getDest();
+  Value* cpySrc = cpy->getSource();
+  CallSite CS = CallSite::get(C);
+
+  // We need to be able to reason about the size of the memcpy, so we require
+  // that it be a constant.
+  ConstantInt* cpyLength = dyn_cast<ConstantInt>(cpy->getLength());
+  if (!cpyLength)
+    return false;
+
+  // Require that src be an alloca.  This simplifies the reasoning considerably.
+  AllocaInst* srcAlloca = dyn_cast<AllocaInst>(cpySrc);
+  if (!srcAlloca)
+    return false;
+
+  // Check that all of src is copied to dest.
+  TargetData& TD = getAnalysis<TargetData>();
+
+  ConstantInt* srcArraySize = dyn_cast<ConstantInt>(srcAlloca->getArraySize());
+  if (!srcArraySize)
+    return false;
+
+  uint64_t srcSize = TD.getABITypeSize(srcAlloca->getAllocatedType()) *
+    srcArraySize->getZExtValue();
+
+  if (cpyLength->getZExtValue() < srcSize)
+    return false;
+
+  // Check that accessing the first srcSize bytes of dest will not cause a
+  // trap.  Otherwise the transform is invalid since it might cause a trap
+  // to occur earlier than it otherwise would.
+  if (AllocaInst* A = dyn_cast<AllocaInst>(cpyDest)) {
+    // The destination is an alloca.  Check it is larger than srcSize.
+    ConstantInt* destArraySize = dyn_cast<ConstantInt>(A->getArraySize());
+    if (!destArraySize)
+      return false;
+
+    uint64_t destSize = TD.getABITypeSize(A->getAllocatedType()) *
+      destArraySize->getZExtValue();
+
+    if (destSize < srcSize)
+      return false;
+  } else if (Argument* A = dyn_cast<Argument>(cpyDest)) {
+    // If the destination is an sret parameter then only accesses that are
+    // outside of the returned struct type can trap.
+    if (!A->hasStructRetAttr())
+      return false;
+
+    const Type* StructTy = cast<PointerType>(A->getType())->getElementType();
+    uint64_t destSize = TD.getABITypeSize(StructTy);
+
+    if (destSize < srcSize)
+      return false;
+  } else {
+    return false;
+  }
+
+  // Check that src is not accessed except via the call and the memcpy.  This
+  // guarantees that it holds only undefined values when passed in (so the final
+  // memcpy can be dropped), that it is not read or written between the call and
+  // the memcpy, and that writing beyond the end of it is undefined.
+  SmallVector<User*, 8> srcUseList(srcAlloca->use_begin(),
+                                   srcAlloca->use_end());
+  while (!srcUseList.empty()) {
+    User* UI = srcUseList.back();
+    srcUseList.pop_back();
+
+    if (isa<GetElementPtrInst>(UI) || isa<BitCastInst>(UI)) {
+      for (User::use_iterator I = UI->use_begin(), E = UI->use_end();
+           I != E; ++I)
+        srcUseList.push_back(*I);
+    } else if (UI != C && UI != cpy) {
+      return false;
+    }
+  }
+
+  // Since we're changing the parameter to the callsite, we need to make sure
+  // that what would be the new parameter dominates the callsite.
+  DominatorTree& DT = getAnalysis<DominatorTree>();
+  if (Instruction* cpyDestInst = dyn_cast<Instruction>(cpyDest))
+    if (!DT.dominates(cpyDestInst, C))
+      return false;
+
+  // In addition to knowing that the call does not access src in some
+  // unexpected manner, for example via a global, which we deduce from
+  // the use analysis, we also need to know that it does not sneakily
+  // access dest.  We rely on AA to figure this out for us.
+  AliasAnalysis& AA = getAnalysis<AliasAnalysis>();
+  if (AA.getModRefInfo(C, cpy->getRawDest(), srcSize) !=
+      AliasAnalysis::NoModRef)
+    return false;
+
+  // All the checks have passed, so do the transformation.
+  for (unsigned i = 0; i < CS.arg_size(); ++i)
+    if (CS.getArgument(i) == cpySrc) {
+      if (cpySrc->getType() != cpyDest->getType())
+        cpyDest = CastInst::createPointerCast(cpyDest, cpySrc->getType(),
+                                              cpyDest->getName(), C);
+      CS.setArgument(i, cpyDest);
+    }
+
+  // Drop any cached information about the call, because we may have changed
+  // its dependence information by changing its parameter.
+  MemoryDependenceAnalysis& MD = getAnalysis<MemoryDependenceAnalysis>();
+  MD.dropInstruction(C);
+
+  // Remove the memcpy
+  MD.removeInstruction(cpy);
+  toErase.push_back(cpy);
+
+  return true;
+}
+
+/// processMemCpy - perform simplication of memcpy's.  If we have memcpy A which
+/// copies X to Y, and memcpy B which copies Y to Z, then we can rewrite B to be
+/// a memcpy from X to Z (or potentially a memmove, depending on circumstances).
+///  This allows later passes to remove the first memcpy altogether.
+bool MemCpyOpt::processMemCpy(MemCpyInst* M, MemCpyInst* MDep,
+                        SmallVectorImpl<Instruction*> &toErase) {
+  // We can only transforms memcpy's where the dest of one is the source of the
+  // other
+  if (M->getSource() != MDep->getDest())
+    return false;
+  
+  // Second, the length of the memcpy's must be the same, or the preceeding one
+  // must be larger than the following one.
+  ConstantInt* C1 = dyn_cast<ConstantInt>(MDep->getLength());
+  ConstantInt* C2 = dyn_cast<ConstantInt>(M->getLength());
+  if (!C1 || !C2)
+    return false;
+  
+  uint64_t DepSize = C1->getValue().getZExtValue();
+  uint64_t CpySize = C2->getValue().getZExtValue();
+  
+  if (DepSize < CpySize)
+    return false;
+  
+  // Finally, we have to make sure that the dest of the second does not
+  // alias the source of the first
+  AliasAnalysis& AA = getAnalysis<AliasAnalysis>();
+  if (AA.alias(M->getRawDest(), CpySize, MDep->getRawSource(), DepSize) !=
+      AliasAnalysis::NoAlias)
+    return false;
+  else if (AA.alias(M->getRawDest(), CpySize, M->getRawSource(), CpySize) !=
+           AliasAnalysis::NoAlias)
+    return false;
+  else if (AA.alias(MDep->getRawDest(), DepSize, MDep->getRawSource(), DepSize)
+           != AliasAnalysis::NoAlias)
+    return false;
+  
+  // If all checks passed, then we can transform these memcpy's
+  Function* MemCpyFun = Intrinsic::getDeclaration(
+                                 M->getParent()->getParent()->getParent(),
+                                 M->getIntrinsicID());
+    
+  std::vector<Value*> args;
+  args.push_back(M->getRawDest());
+  args.push_back(MDep->getRawSource());
+  args.push_back(M->getLength());
+  args.push_back(M->getAlignment());
+  
+  CallInst* C = CallInst::Create(MemCpyFun, args.begin(), args.end(), "", M);
+  
+  MemoryDependenceAnalysis& MD = getAnalysis<MemoryDependenceAnalysis>();
+  if (MD.getDependency(C) == MDep) {
+    MD.dropInstruction(M);
+    toErase.push_back(M);
+    return true;
+  }
+  
+  MD.removeInstruction(C);
+  toErase.push_back(C);
+  return false;
+}
+
+/// processInstruction - When calculating availability, handle an instruction
+/// by inserting it into the appropriate sets
+bool MemCpyOpt::processInstruction(Instruction *I,
+                                   SmallVectorImpl<Instruction*> &toErase) {
+  if (StoreInst *SI = dyn_cast<StoreInst>(I))
+    return processStore(SI, toErase);
+  
+  if (MemCpyInst* M = dyn_cast<MemCpyInst>(I)) {
+    MemoryDependenceAnalysis& MD = getAnalysis<MemoryDependenceAnalysis>();
+
+    // The are two possible optimizations we can do for memcpy:
+    //   a) memcpy-memcpy xform which exposes redundance for DSE
+    //   b) call-memcpy xform for return slot optimization
+    Instruction* dep = MD.getDependency(M);
+    if (dep == MemoryDependenceAnalysis::None ||
+        dep == MemoryDependenceAnalysis::NonLocal)
+      return false;
+    if (MemCpyInst *MemCpy = dyn_cast<MemCpyInst>(dep))
+      return processMemCpy(M, MemCpy, toErase);
+    if (CallInst* C = dyn_cast<CallInst>(dep))
+      return performCallSlotOptzn(M, C, toErase);
+    return false;
+  }
+  
+  return false;
+}
+
+// MemCpyOpt::runOnFunction - This is the main transformation entry point for a
+// function.
+//
+bool MemCpyOpt::runOnFunction(Function& F) {
+  
+  bool changed = false;
+  bool shouldContinue = true;
+  
+  while (shouldContinue) {
+    shouldContinue = iterateOnFunction(F);
+    changed |= shouldContinue;
+  }
+  
+  return changed;
+}
+
+
+// MemCpyOpt::iterateOnFunction - Executes one iteration of GVN
+bool MemCpyOpt::iterateOnFunction(Function &F) {
+  bool changed_function = false;
+  
+  DominatorTree &DT = getAnalysis<DominatorTree>();   
+  
+  SmallVector<Instruction*, 8> toErase;
+
+  // Top-down walk of the dominator tree
+  for (df_iterator<DomTreeNode*> DI = df_begin(DT.getRootNode()),
+         E = df_end(DT.getRootNode()); DI != E; ++DI) {
+
+    BasicBlock* BB = DI->getBlock();
+    for (BasicBlock::iterator BI = BB->begin(), BE = BB->end();
+         BI != BE;) {
+      changed_function |= processInstruction(BI, toErase);
+      if (toErase.empty()) {
+        ++BI;
+        continue;
+      }
+      
+      // If we need some instructions deleted, do it now.
+      NumMemCpyInstr += toErase.size();
+      
+      // Avoid iterator invalidation.
+      bool AtStart = BI == BB->begin();
+      if (!AtStart)
+        --BI;
+
+      for (SmallVector<Instruction*, 4>::iterator I = toErase.begin(),
+           E = toErase.end(); I != E; ++I)
+        (*I)->eraseFromParent();
+
+      if (AtStart)
+        BI = BB->begin();
+      else
+        ++BI;
+      
+      toErase.clear();
+    }
+  }
+  
+  return changed_function;
+}